Various types of tire pressure sensors have been developed that are situated within tires of vehicles (see, e.g., U.S. Pat. Nos. 5,540,092 and 5,987,980). These tire pressure sensors require electrical power to monitor and transmit information regarding the current tire pressure for display in the passenger compartment of the vehicle. Typically, the tire pressure sensors are powered by long-life batteries also disposed within the tires. The lifetime of these long-life batteries, however, is five to ten years, which is less than the life span of many automobiles and trucks. The effective lifetime of these batteries may be even shorter, because it is not desirable to run the battery down to such an extent that the tire pressure sensor system fails before the battery is replaced. Furthermore, replacing a tire pressure sensor battery is a labor-intensive process, involving, e.g., the removal of the tire from the wheel.
Accordingly, some tire pressure systems attempt to extend the life of the battery by providing a centrifugal switch disposed between the tire pressure sensor and the battery. This centrifugal switch is activated only when the tires rotate beyond a certain rate. As a result, the battery is used for monitoring tire pressure only when the vehicle is in motion. Another approach to extend the life of the battery is to limit the transmission of the tire pressure information only to certain conditions, for example, if there is a drop in tire pressure or if the tire pressure information is requested by the driver.
These approaches, however, are difficult to model statistically and therefore guarantee a reliable battery lifetime that is commensurate with the life span of the vehicle under a wide range of uses. For example, vehicles in interstate trucking are in motion to a much greater extent than pleasure or commuting vehicles.
Another problem with these approaches is that the usefulness of the tire pressure sensor is curtailed. For example, when the centrifugal switch is used, the tire pressure can only be sensed when the vehicle is in motion. As another example, when the transmission of tire pressure information is limited to certain, rare conditions, the driver of the vehicle cannot be provided with real-time information on the pressure of the vehicle's tires.
Therefore, there is a need for extending the life of a battery of a tire pressure sensor and for avoid the necessity of replacing a tire pressure sensor battery. There is also need for making tire pressures useful in more situations, e.g. when the vehicle is standing still.